Generally, handover (HO) means that a mobile station moves away from a radio interface of one base station into a radio interface of another base station. In the following description, a handover procedure in a general IEEE 802.16 system is explained.
In IEEE 802.16 network, in order to inform a mobile station (hereinafter abbreviated MS) of information (e.g., topology) on a basic network configuration, a serving base station (hereinafter abbreviated SBS) is able to broadcast neighbor base station information via a neighbor advertisement (MOB_NBR-ADV) message.
In the MOB_NBR-ADV message, contained are such system information on a serving base station and neighbor base stations as a preamble index, a frequency and a handover (HO) optimization availability, DCD/UCD (downlink channel descriptor/uplink channel descriptor) information and the like.
The DCD/UCD information contains informations that should be known to a mobile station in order for the mobile station to perform information communications in DL/UL (downlink/uplink). For instance, the informations may include handover (HO) trigger information, MAC (medium access control) version information of a base station, MIH (media independent handover) capability information of a base station and the like.
In a general MOB_NBR-ADV message, informations on neighbor base stations of IEEE 802.16e type are contained. Hence, informations of neighbor base stations of types except UEEE 802.16e may be broadcasted to mobile stations via SII-ADV (service identity information advertisement) message. Accordingly, the mobile station may be able to acquire informations of a heterogeneous network base station by making a request for a serving base station to send the SII-ADV message.
In the following description, a procedure for a mobile station, which has acquired information on a neighbor base station by the above-described method, to perform a handover on IEEE 802.16 network is explained in detail with reference to FIG. 1.
FIG. 1 shows one example of a handover procedure executable in a general IEEE 802.16 system.
Referring to FIG. 1, first of all, a mobile station (MS) is connected with a serving base station (SBS) and is then able to perform a data exchange [S101].
The serving base station periodically broadcasts information on a neighbor base station neighboring to the serving base station to the mobile station via MOB_NBR-ADV message [S102].
In the course of communicating with the serving base station, the mobile station is able to start scanning candidate base stations (candidate HO BSs) using a handover trigger condition. If such a handover condition as a prescribed hysterisis margin is exceeded, the mobile station is able to make a request for a handover procedure execution to the serving base station by sending a handover request (MOB_MSHO-REQ) message [S103].
The serving base station is able to notify the handover request made by the mobile station to the candidate base stations (candidate HO BSs) contained in the MOB_MSHO-REQ message [S104].
Each of the candidate base stations (candidate HO BSs) takes a preemptive step for the mobile station having made the request for the handover and is then able to deliver informations related to the handover to the serving base station via HO-RSP message [S105].
The serving base station is able to deliver the handover related informations, which are obtained from the candidate base stations (candidate HO BSs) via the HO-RSP messages, to the mobile station via handover response (MOB_BSHO-RSP) message. In this case, in the MOB_BSHO-RSP message, such information for performing the handover as an action time for the handover, a handover identifier (HO-ID), a dedicated handover (HO) CDMA ranging code and the like can be included [S106].
Based on the information included in the MOB_BSHO-RSP message received from the serving base station, the mobile station is able to determine a target base station among the candidate base stations. Subsequently, the mobile station is able to attempt a ranging by transmitting CDMA code to the determined target base station [S107].
Having received the CDMA code, the target base station is able to transmit a success or failure of the ranging and physical correction values to the mobile station via a ranging response (RNG-RSP) message [S108].
Subsequently, the mobile station is able to send a ranging request (RNG-REQ) message for authentication to the target base station [S109].
Having received the ranging request message, the target base station is able to provide the mobile station with such system information usable by the corresponding base station as CID (connection identifier) and the like via a ranging response message [S110].
If the target base station successfully completes the authentication of the mobile station and sends all update information, it may be able to notify a success or failure of the handover to the serving base station of the mobile station via a handover complete message (HO-CMPT) [S111].
Thereafter, the mobile station is able to perform information exchange with the handover performed base station [S112].
The above-described handover process is assumed as performed between a mobile station and a base station in accordance with IEEE 902.16e specification (e.g.; WirelessMAN-OFDMA R1 Reference System). For clarity of the following description, a system, to which a general technology including IEEE 802.16e specification is applied, is named ‘legacy system’. And, a mobile station, to which a legacy technology is applied, is named ‘VMS (yardstick MS)’ or a ‘legacy mobile station’. Moreover, a base station, to which a legacy technology is applied, is named ‘legacy base station’, ‘R1 BS’ or ‘YBS (yardstick BS)’.
A mobile station, to which a technology (e.g., IEEE 802.16m specification: WirelessMAN-OFDMA Advanced System) further advanced than a general technology is applied, is named ‘AMS (advanced MS)’ or ‘advanced mobile station’. And, a base station, to which the advanced technology is applied, is named ‘ABS (advanced BS)’ or ‘advanced base station’.
Assume a case that AMS is receiving a service by being connected with YBS and that AMS (WirelessMAN-OFDMA R1 Reference System/VVirelessMAN-OFDMA Advanced Co-existing system) supporting both AMS and YMS exists by neighboring to the YBS.
Assume that the YBS ha a legacy zone (LZone) having a physical channel frame structure applied to a legacy system. Assume that the ABS has an advanced mobile station support zone (MZone: 16M zone) having a physical channel frame structure applied to an AMS supportive only advanced system (WirelessMAC-OFDMA advanced system only). Assume that ABS supporting both AMS and YMS (WirelessMAC-OFDMA Reference System/WirelessMAC-OFDMA Advanced Co-existing System: legacy supportive) has a legacy zone and an advanced mobile station supportive zone and that each zone is divided by time unit, e.g., by frame or subframe unit (TDD: time division duplex) in uplink and downlink each.
Assume that AMS is able to receive services from ABS and YBS both. In particular, assume that the AMS is able to receive a service via one of an advanced mobile station supportive zone and a legacy zone and that the AMS is able to perform both a handover performing process defined in a legacy system and a handover performing process defined in an advanced system.
Generally, in order to perform a handover into ABS supporting both AMS and YMS from a serving YBS, AMS firstly enters a legacy zone of the ABS and then keeps receiving a service or may perform a zone switch to an advanced mobile station supportive zone. Moreover, the AMS may be able to perform a handover in a manner of switching a zone into an advanced mobile station supportive zone directly without entering the legacy zone of the ABS.
In particular, regarding the zone switch, as LZone and MZone divided by time division duplex (TDD) exist within a single carrier, AMS used to operate in the LZone is set to operate in a resource region of the MZone. In more particular, IEEE 802.16m MAC operation is performed by being switched from IEEE 802.16e MAC operation. This may include an operation that the AMS moves to the LZone from the MZone.
For this zone switch, when AMS performs a handover into ABS supporting both YMS and AMS from YBS, in order for the AMS to receive a service according to an advanced system specification from the ABS, the AMS needs to inform the ABS that it is the AMS itself. Moreover, if the ABS recognizes the handover of the AMS, it should transmit system information of the ABS for the enhanced system, i.e., system information on an advanced mobile station supportive zone to the AMS. Besides, in order to receive a service according to the advanced system specification of the ABS, the AMS should attempt a ranging to the advanced mobile station supportive zone. To this end, the AMS needs to make a request for a bandwidth.
Therefore, in order for a mobile station following an advanced system specification to perform a handover, the demand for an efficient handover method with backward compatibility is rising.